LTC1876 datasheet(12/36 Pages) LINER

12 / 36 page

LTC1876

1876fa

cycles, followed by a variable “sleep” interval depending

upon the load current. The resultant output voltage ripple

is held to a very small value by having the hysteretic

comparator after the error amplifier gain block.

Constant Frequency Operation

When the FCB pin is tied to INTVCC, Burst Mode operation

is disabled and the forced minimum output current re-

quirement is removed. This provides constant frequency,

discontinuous (preventing reverse inductor current) cur-

rent operation over the widest possible output current

range. This constant frequency operation is not as efficient

as Burst Mode operation, but does provide a lower noise,

constant frequency operating mode down to approxi-

mately 1% of designed maximum output current.

Constant Current (PWM) Operation

Tying the FCB pin to ground will force continuous current

operation. This is the least efficient operating mode, but

may be desirable in certain applications. The output can

source or sink current in this mode. When sinking current

while in forced continuous operation, current will be

forced back into the main power supply potentially boost-

ing the input supply to dangerous voltage levels—

BEWARE!

Frequency Setting

The FREQSET pin provides frequency adjustment to the

controllers’ internal oscillator from approximately 140kHz

to 310kHz. This input is nominally biased through an

internal resistor to the 1.19V reference, setting the oscil-

lator frequency to approximately 220kHz. This pin can be

driven from an external AC or DC signal source to control

the instantaneous frequency of the oscillator. The auxillary

boost regulator operates at a constant 1.2MHz frequency.

INTVCC/EXTVCC Power

Power for the top and bottom MOSFET drivers and most

other internal circuitry is derived from the INTVCC pin.

When the EXTVCC pin is left open, an internal 5V low

dropout linear regulator supplies INTVCC power. If EXTVCC

is taken above 4.7V, the 5V regulator is turned off and an

internal switch is turned on connecting EXTVCC to INTVCC.

This allows the INTVCC power to be derived from a high

efficiency external source such as the output of the regu-

lator itself or a secondary winding, as described in Appli-

cations Information.

Standby Mode Pin

The STBYMD pin is a three-state input that controls

common circuitry within the IC as follows: When the

STBYMD pin is held at ground, both controller RUN/SS

pins are pulled to ground providing a single control pin to

shut down both controllers. When the pin is left open, the

internal RUN/SS currents are enabled to charge the

RUN/SS capacitor(s), allowing the turn-on of either con-

troller and activating necessary common internal biasing.

When the STBYMD pin is taken above 2V, both internal

linear regulators are turned on independent of the state of

the two switching regulator controllers, providing output

power to “wake-up” other circuitry. Decouple the pin with

a small capacitor (0.01

µF) to ground if the pin is not

connected to a DC potential.

Output Overvoltage Protection

An overvoltage comparator, OV, guards against transient

overshoots (>7.5%) as well as other more serious condi-

tions that may overvoltage the output. In this case, the top

MOSFET is turned off and the bottom MOSFET is turned on

until the overvoltage condition is cleared.

Power Good (PGOOD) Pin

The PGOOD pin is connected to an open drain of an internal

MOSFET. The MOSFET turns on and pulls the pin low when

both the outputs are not within

±7.5% of their nominal

output levels as determined by their resistive feedback

dividers. When both controller outputs meet the

±7.5%

requirement, the MOSFET is turned off within 10

µsandthe

pin is allowed to be pulled up by an external resistor to a

source of up to 7V. The auxiliary regulator’s output is not

monitored.

Foldback Current, Short-Circuit Detection and Short-

Circuit Latchoff

The RUN/SS capacitors are used initially to limit the inrush

current of each step-down switching regulator. After the

(Refer to Functional Diagram)

OPERATIO

LTC1876 Datasheet(PDF) 12 Page - Linear Technology

LTC1876 Datasheet(HTML) 12 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

Datasheet Download

Link URL